The first atomic bomb--the Little Boy dropped on Hiroshima--was already obsolete when it was used. During the course of the Cold War, American nuclear weapons evolved from plutonium-fission bombs with 12 kilotons of yield, to thermonuclear hydrogen bombs with several megatons of power.
Here is a photo-history of American nuclear weapons, on display at the US Air Force Museum in Dayton OH..
Little Boy. The "gun" mechanism used to assemble the critical mass was inefficient but simple. The US was so sure it would work they never even tested it--the prototype was used in combat. The explosive yield was estimated at around 12 kilotons.
Fat Man. The implosion plutonium bomb was a lot more complex than Little Boy, so the US tested it in the Trinity explosion in New Mexico. Fat Man became the basis for the US arsenal, and also was stolen and copied by the Soviets. It had a slightly higher yield than Little Boy, perhaps 15 kilotons.
Mark 6 nuclear bomb. A modified version of Fat Man produced from 1951 to 1955. It used a higher number of detonators and a thinner layer of conventional explosives, and was about 25% lighter than Fat Man. It could produce a yield up to 160 kilotons.
Mark 7 nuclear bomb. This was a tactical nuclear bomb designed to be carried by fighter-bombers. It weighed only 600 pounds. The plutonium implosion could be controlled by using different components, allowing the yield to be set between 10 and 60 kilotons.
Mark 17 nuclear bomb. The first operational thermonuclear fusion bomb, or "hydrogen bomb", it was deployed in 1954. The bomb used an Ulam-Teller configuration, in which a plutonium fission bomb was first detonated, producing radiation which compressed a container of fusion fuel (lithium deuteride), which then triggered fission in a layer of unenriched uranium. The yield was about 12 megatons. The Mark 17, at almost 25 feet long and 20 tons, was the largest nuclear weapon ever deployed by the US.
Mark 39 nuclear bomb. Deployed from 1957 to 1966, the Mark 39 was another Ulam-Teller configuration with a 4 megaton yield. Weighing a little over 6,000 pounds, it was intended for use as a missile warhead as well as an air-dropped bomb.
Mark 41 nuclear bomb. Deployed in 1961 and used throughout the 60's, the Mark 41 was the only three-stage Ulam-Teller design deployed by the US. It used an extra fusion stage to push the yield up to 25 megatons, the most powerful nuclear bomb in the arsenal. Only the B-47 and the B-52 aircraft were capable of carrying it.
Mark 53 nuclear bomb. Introduced in 1962, the air-dropped bomb version was known as the B-53. It was intended to hit hardened Soviet targets like underground command bunkers. A small number of B-53 bombs remained in the US arsenal until 2010.
W-53 warhead. The ICBM version of the B-53 aerial bomb, used in the Titan II nuclear missile. It had a yield of 9 megatons.
MIRV warheads, MX Peacekeeper Missile. Deployed in 1986. The Peacekeeper ICBM used multiple independently-targeted re-entry vehicles (MIRV), in which ten separate warheads could be launched on one missile, each warhead aimed at a different target. (The earlier Minuteman III missile had three MIRVS of 400 kilotons.) The warheads were W87 thermonuclear devices with a yield of about 300 kilotons. All of the Peacekeeper missiles were destroyed by treaty in 2005.
Air-launched cruise missile. Cruise missiles were first deployed in the early 1980s, as air-launched (ALCMs), ground-launched (GLCMs) and submarine-launched (SLCMs) versions. They were designed to penetrate Soviet air defenses and deliver a W80 nuclear warhead with a yield configured between 10 and 150 kilotons.